Where are you looking for the "write off" amount? Business expenses are subtracted from your business income, not from your tax. Entering the expenses as Supplies will reduce your business income by 100% of the amount you enter. It will not reduce your tax by 100% of the expenses. The reduction in your tax depends on what tax bracket you are in.
For example, if you charged a customer $5,000 for a job, and you paid $1,000 for materials, your profit from the job is $4,000. You pay tax on $4,000 of income. Your taxable income was reduced by 100% of the cost of the materials.
